Polyurethane coatings are extensively used as commercial and industrial protective and/or decorative coatings. Polyurethane coatings, known in the industry as one of the toughest coatings available, are routinely applied as protective coatings on exterior walls of buildings, industrial machinery, military equipment and vehicles, commercial and passenger vehicles, and any other surface requiring a protective coating. Polyurethane systems are also used extensively as sealants and adhesives.
Protective coatings, as the name suggests, protect the surface on which it is applied from destructive environmental conditions. These conditions include exposure to ultraviolet radiation (UV), acid rain, salt, chipping agents such as stones and hail, and extremes in temperature and humidity. Protection of the substrate surfaces is particularly important in geographical areas with harsh environmental conditions. For example, in arid regions, substrates and coatings are exposed to high amounts of UV radiation. In other geographical regions, substrates and coatings are exposed to ocean spray and/or road salt, or acid rain.
A survey of the effects of environmental conditions on automotive coatings was described by David R. Bauer in J. Coat. Tech., 66:835, 57-65, (1994). UV damage occurs primarily through free radical oxidation. When a coating absorbs UV radiation, a free radical is generated and combines with oxygen to produce a peroxy radical. The peroxy free radical then abstracts a hydrogen from the coating creating a carbon based radical. The hydrogen abstraction propagates until two free radicals combine in a termination step. UV damage of coatings result in loss of film properties, including loss of gloss. Ultimately, degradation of the coating is the final product of UV exposure.
In all of the diverse geographic regions, the coating must be able to withstand these environmental conditions if it is to protect the substrate. Consequently, there is a need for coatings with improved physical properties including resistance to UV radiation, impact resistance, salt resistance, acid resistance, organic chemical resistance, chip resistance, abrasion resistance, downglossing minimization, and bubble elimination.
Downglossing occurs when polyurethane coatings are applied under conditions of high humidity. This phenomenon is caused by the reaction of water with isocyanate, resulting in microbubbles on the surface of the coating which reduce gloss. Downglossing affects air dry systems more severely than forced dry or baked systems. There is a need for an agent which can minimize downglossing in coating systems.
When water reacts with isocyanate within a preparation, it forms bubbles which create pinholes within the preparation. Bubbles have been visually eliminated from the coating, improving the film integrity, by providing improved abrasion resistance and impact resistance. An agent that can minimize or visually eliminate bubble formation in a coating, adhesive or sealant is also required.
Furthermore, for a coating to be useful, it must have a reasonable pot life to provide a sufficient time for application of the coating and to provide for longer storage time. U.S. Pat. No. 4,504,647 describes aldimine oxazolidine compounds as curing agents which exhibit a long shelf life when mixed with isocyanate. However, the mixture cures very quickly into an elastic or hard polymer in the presence of water or humidity. Coatings require an adequate pot life in order to be used commercially as automotive refinish coatings and the like. Coatings such as those described in U.S. Pat. No. 4,504,647 rapidly react with water to form a gel. These coatings have a short pot life which can prevent use of the coating formulation in typical commercial applications. There is a need for methods and compositions for improving the pot life of coatings.
Polyurethane coating systems typically include a polyisocyanate or prepolymer component and/or a polyol which react to form the polyurethane film. Alternatively, a polyurethane coating can include only the polyisocyanate component or the prepolymer component. When only the polyisocyanate or prepolymer component is included, polyisocyanate or prepolymer molecules cross-react with each other and/or with moisture to form a useful coating.
As used herein, a polyisocyanate is an isocyanate polymer; a polyol is a molecule with multiple hydroxyl groups; and a prepolymer is a polyisocyanate with covalently attached polyols with excess isocyanate groups. Exemplary polyols include polyester polyol, polyether polyol and acrylic polylol. In some polyurethane coating systems, the polyisocyanate component is blocked to prevent the polyisocyanate from reacting with the polyol prematurely. Upon exposure to deblocking conditions, the blocking groups are removed and the polyisocyanate reacts with the polyol to form the useful coating. Polyurethane coating systems also include pigments, volatile organic solvents, and a variety of adjuvant components, e.g., surface active agents, dispersants, diluents, and fillers.
Moisture curing polyurethane coating systems include a polyisocyanate or a prepolymer component which reacts with atmospheric water at room temperature to form useful films. These systems also include pigments, organic solvents, and a variety of adjuvant components, e.g., surface active agents, dispersants, diluents, and fillers. Since the polyisocyanate component reacts with even trace amounts of moisture, extreme care must be taken so that the polyisocyanates do not contact water until they are applied to a surface to be coated. Water is, however, unintentionally and unavoidably introduced into the formulation process in the form of dissolved water in solvents, adsorbed and absorbed moisture on the surfaces of fillers and pigments, and atmospheric moisture. Subsequent reaction of the water with the polyisocyanate component of the system results in an irreversible reaction which will harden the product, making it unusable before it can be applied to the surface to be coated. This water must be removed in order to produce an acceptable product. The existing methods for preparing color-pigmented moisture curable polyurethane coatings require expensive equipment to dry the pigments, solvents, and fillers. In the alternative, moisture scavenging agents are added to the coating preparation or are added to the pigments, solvents and other raw materials prior to preparing the coating.
One group of moisture scavenging agents widely used to prevent moisture contamination of moisture curable polyurethane coating systems is the monomeric isocyanates. A typical monomeric isocyanate, such as p-toluenesulfonyl isocyanate (Vanchem, Inc. Lockport, Conn.), reacts with water to generate carbon dioxide and the corresponding sulfonamide, e.g., p-toluenesulfonamide. The carbon dioxide diffuses from the pigment grind during the dehydration phase as carbon dioxide gas. A disadvantage of monomeric isocyanates is that they are harmful if swallowed, inhaled, or absorbed through the skin and are extremely corrosive to the tissues of the mucous membranes, upper respiratory tract, and skin.
There is a need for a moisture scavenger which efficiently, cost effectively, and safely removes moisture from moisture curable coating systems and from any other preparation where residual water is a problem, without seriously detracting from the performance of the cured coating.
Currently, there are available commercially two types of polyurethane systems, a one component system (1-K), and a two component system (2-K). Both the 1-K and the 2-K systems are widely used in commercial applications depending upon the specific needs of the coating system.
In the two component system, the polyisocyanate and the polyol are segregated into two separate containers until use. When the two components are mixed together, the coating cures in which the polyisocyanate cross-links with the polyol to form the polyurethane coating.
In the one component system, the polyisocyanate or prepolymer and/or the polyol are both included in one package. Alternatively, the one component includes a blocked polyisocyanate and polyol in one package. To prevent premature curing, the polyisocyanate is chemically blocked or is in the form of a prepolymer. Upon exposure to deblocking conditions (moisture, heat or radiation, etc.), the blocking group is removed and the polyurethane coating cures into a film. Well known blocking groups of the polyisocyanate include diethyl malonate, 3,5-dimethylpyrazole, MEK Oxime, .epsilon.-caprolactam and 2-ethyl hexanol. While the one component system is convenient to use, the blocked polyisocyanates are either very viscous or are solids. As a result, one component polyurethane systems require large amounts of volatile organic solvents to reduce the viscosity or to solubilize the individual components. Typically, one gallon of a one component polyurethane system contains about four to five pounds of volatile organic compounds.